Site amplification in shallow soils in Kashiwazaki city, estimated from ground motion data from aftershocks of the 2207 chuetuoki earthquake and microtremor explorations

Observations of earthquake ground motion due to aftershocks of the 2007 Chuuetsu-oki earthquake were conducted in Kashiwazaki city, Niigata prefecture, by installing 10 seismometers with different geological conditions for aiming at understanding local site amplification in the area. The site amplification shows large peaks around one second at the site in the central parts of the city. We also conducted microtremor array explorations at the observation sites. The shallow Vs-profiles were estimated from inversions of Rayleigh wave phase velocities obtained from array analysis of vertical microtremors. The amplifications of S-waves for the inverted profiles are similar to those from the aftershock ground motions. The average S-wave velocity for the top 30 meters of the S-wave profiles is calculated at each site. The average velocities at the sites with damage of wooden houses are less than 130 m/s.